Ground penetrating radar is known for use in detecting underground structure such as pipes or the like. In such systems, microwave frequency radio (radar) signals are transmitted into the earth, and echoes off sub-surface structures are detected and displayed.
In order to increase the signal strength of the returning echo, the microwave transmitting antenna is typically placed close to the surface of the earth to direct energy directly downward. The antenna may be attached to the back of a truck that is driven over the surface of the site to be investigated. Multiple readings or a continuous band of readings may be obtained and a plot produced in which movement of the antenna is plotted in the x-axis and the echo signal is plotted in the y-axis. Stronger echo signals are represented by a darker shading so that a pipe or similar echo producing structure may be identified.
The images produced by these techniques can be of low quality because of the difficulty of coupling adequate radar energy into the earth. The need to move the antenna over the surface of the earth is cumbersome and in many important applications may be difficult or impossible.